1. Field of the Invention
The present invention relates to a method for producing alkali cellulose and a method for producing cellulose ether by using the alkali cellulose.
2. Description of the Related Art
For the preparation of cellulose ether, known is a method of bringing an alkali solution into contact with a highly purified pulp to prepare the corresponding alkali cellulose, and etherifying the resulting alkali cellulose with an etherifying agent. The cellulose ether obtained as a final product becomes water-soluble by properly controlling its degree of substitution. The water-soluble cellulose ether contains a water-insoluble portion which sometimes lowers the light transmittance of the aqueous solution of the cellulose ether or damages the commodity value of the cellulose ether as a contaminant. This insoluble portion appears to be caused by the presence of a low-substituted portion which does not have enough substituents to dissolve in water. One of the reasons why the insoluble portion is present is that an alkali distribution is uneven in the alkali cellulose.
Functions of this alkali include swelling the cellulose therewith to change its crystal structure in the pulp and thereby promoting the penetration of the etherifying agent; catalyzing the etherification reaction with an alkylene oxide; and serving as a reactant for an alkyl halide. A portion of pulp not brought into contact with the aqueous alkali solution is not involved in the reaction and therefore remains as an undissolved portion. Lack of uniformity of the alkali cellulose directly leads to the undissolved portion.
To produce alkali cellulose, JP 2007-197682A describes a method comprising the steps of continuously bringing pulp chips into contact with an alkaline metal hydroxide solution having a concentration of 23 to 60% by weight to obtain a contact product, and draining the contact product with a continuous centrifugal separator.